Simulating the hydrological performance of low impact development in shallow groundwater via a modified SWMM

Low impact development (LID) practices are effective in restoring natural hydrological processes, but they may be affected by groundwater and pose additional risk to groundwater dynamics and contamination in shallow groundwater conditions. Their performance in shallow groundwater is not well understood due to monitoring difficulty and the deficiency of modeling tools. The Storm Water Management Model (SWMM) is an urban hydrology-hydraulic model that has been widely used to evaluate the performance of LID practices. However, it cannot accurately simulate the hydrological processes of LID practices in shallow groundwater. This study presents the development, calibration, validation, and testing of a modified SWMM (SWMM-LID-GW) that incorporates groundwater feedbacks into LID simulations. The hydrological performance of LID practices under different environmental conditions was simulated, based on which the impacts of initial groundwater table depth, rainfall type, and in-situ soil type were evaluated, and a number of general nonlinear multivariate formulas were proposed to predict the hydrologic performance of LID practices in shallow groundwater environment and support the feasibility analysis of LID practices before project implementation. The reduction of exfiltration and increase of groundwater drainage in shallow groundwater accounted for a significant proportion of the water budget of LID practices. The influence of shallow groundwater was greater for shallower groundwater tables, in less permeable in-situ soils, and for events of higher intensities and longer durations. However, due to the dominant role of shallow groundwater, the impacts of both rainfall type and in-situ soil type on the hydrological performance of LID practices in shallow groundwater were minimal in general, in contrast to the impacts on deep groundwater tables. The SWMM-LID-GW outperformed the current SWMM for a wide range of groundwater conditions, particularly for locations in which the groundwater table frequently rises to 1.0 m or less below ground.